Modeling alkali alanates for hydrogen storage by density-functional band-structure calculations
Modeling alkali alanates for hydrogen storage by density-functional band-structure calculations
复制标题
DOI：
10.1557/jmr.2005.0397
复制
发表时间：
2005-12
影响因子：
2.7
通讯作者：
O. Løvvik;O. Swang;S. Opalka
中科院分区：
文献类型：
--
作者：
O. Løvvik;O. Swang;S. Opalka
铝酸盐（复合铝酸盐）具有相对高的重量氢密度，是最有前途的固态储氢材料之一。本文采用基态密度泛函能带结构方法计算了纯铝酸盐和混合铝酸盐的晶体结构和电子结构。计算结果与已有的实验数据吻合得很好。比较了纯铝酸盐的性质，指出Li_3AlH_6相相对较高的稳定性是铝酸锂氢化困难的重要原因。碱金属铝酸盐是非金属的，计算出的带隙约为5 eV，四氢和六氢的带隙为2.5-3 eV。碱金属阳离子与铝氢化物复合物之间的键合被确定为离子键合，而复合物内部的键合是极性共价键合。对一系列假设的混合碱金属铝酸盐化合物进行了模拟，发现四种化合物与纯铝酸盐相比是稳定的：LiNa_2AlH_6，K_2LiAlH_6，K_2NaAlH_6和K_2.5Na_0.5AlH_6。没有发现混合碱四氢呋喃是稳定的，这是由不同化合物内的局部配位来解释的。唯一接近国际氢密度目标的碱金属铝化物是NaAlH_4。
The alanates (complex aluminohydrides) have relatively high gravimetric hydrogen density and are among the most promising solid-state hydrogen-storage materials. In this work, the crystal structure and electronic structure of pure and mixed-alkali alanates were calculated by ground-state density-functional band-structure calculations. The results are in excellent correspondence with available experimental data. The properties of the pure alanates were compared, and the relatively high stability of the Li_3AlH_6 phase was pointed out as an important difference that may explain the difficulty of hydrogenating lithium alanate. The alkali alanates are nonmetallic with calculated band gaps around 5 eV and 2.5-3 eV for the tetra- and hexahydrides. The bonding was identified as ionic between the alkali cations and the aluminohydride complexes, while it is polar covalent within the complex. A broad range of hypothetical mixed-alkali alanate compounds was simulated, and four were found to be stable compared to the pure alanates and each other: LiNa_2AlH_6, K_2LiAlH_6, K_2NaAlH_6, and K_2.5Na_0.5AlH_6. No mixed-alkali tetrahydrides were found to be stable, and this was explained by the local coordination within the different compounds. The only alkali alanate that seemed to be close to fulfilling the international hydrogen density targets was NaAlH_4.